Field of the Invention
The invention relates to a steam generator.
In a gas and steam-turbine plant, the heat contained in the expanded working medium or heating gas from the gas turbine is utilized for the generation of steam for the steam turbine. The heat transfer is effected in a waste-heat steam generator, which is disposed down-stream of the gas turbine and in which a number of heating areas for the water preheating, the steam generation and the steam superheating are normally disposed. The heating areas are connected in the water/steam circuit of the steam turbine. The water/steam circuit normally contains several, e.g. three, pressure stages, in which case each pressure stage may have an evaporator heating area.
For the steam generator disposed as a waste-heat steam generator downstream of the gas turbine on the heating-gas side, a number of alternative configuration concepts are suitable, namely the configuration as a once-through steam generator or as a circulation steam generator. In the case of a once-through steam generator, the heating of steam-generator tubes provided as evaporator tubes leads to evaporation of the flow medium in the steam-generator tubes in a single pass. In contrast, in the case of a natural or forced-circulation steam generator, the circulating water is only partly evaporated when passing through the evaporator tubes. The water that is not evaporated in the process is fed again to the same evaporator tubes for further evaporation after separation of the generated steam.
A once-through steam generator, in contrast to a natural or forced-circulation steam generator, is not subject to any pressure limitation. Therefore, live-steam pressures well above the critical pressure of water (P.sub.cri =221 bar), where there is only a slight difference in density between a medium similar to a liquid and a medium similar to steam, are possible. A high live-steam pressure promotes a high thermal efficiency and thus low CO.sub.2 emissions of a fossil-fired power station. In addition, a once-through steam generator has a simple type of construction compared with a circulation steam generator and can therefore be manufactured at an especially low cost. The use of a steam generator configured according to the once-through principle as a waste-heat steam generator of a gas and steam-turbine plant is therefore especially favorable for achieving a high overall efficiency of the gas and steam-turbine plant in a simple type of construction.
A once-through steam generator may in principle, be made in one of two alternative constructional styles, namely in upright type of construction or in horizontal type of construction. Here, a once-through steam generator in a horizontal type of construction is configured for a throughflow of the heating medium or heating gas, for example the exhaust gas from the gas turbine, in an approximately horizontal direction, whereas a once-through steam generator in an upright type of construction is configured for a throughflow of the heating medium in an approximately vertical direction.
A once-through steam generator in the horizontal type of construction, in contrast to a once-through steam generator in the upright type of construction, can be manufactured with especially simple means and at an especially low production and assembly cost. In the case of a once-through steam generator with horizontal type of construction, however, the steam-generator tubes of a heating area, depending on their positioning, are subjected to heating that differs greatly.
In particular in the case of steam-generator tubes leading on the outlet side into a common discharge collector, however, different heating of individual steam-generator tubes may lead to the funneling of steam flows having steam parameters differing greatly from one another and thus to undesirable efficiency losses, in particular to comparatively reduced effectiveness of the relevant heating area and consequently reduced steam generation.
In addition, different heating of adjacent steam-generator tubes, in particular in the region where they lead into a discharge collector, may result in damage to the steam-generator tubes or the collector.